The invention relates to communication systems generally and, more specifically, the invention relates to a low noise block converter method and apparatus suitable for use in a data transmission system.
In information distribution systems utilizing radio frequency (RF) modulation techniques it is well known to modulate each of a plurality of information signals, such as audio, video or data signals onto respective intermediate frequency (IF) carrier signals having a common frequency. The respective IF carrier signals are then modulated onto respective RF carrier frequencies, which are then transmitted to a receiver. The receiver tunes and demodulates an RF carrier frequency including a desired IF modulated information signal. An IF demodulator then retrieves the desired information signal.
Currently in Community Access Television or CATV applications, it is common practice to use Frequency Division Multiplexing (FDM) as a means of inserting a multiplicity of audio, visual or data carriers onto a single cable or optical fiber for transport to the subscriber. That is, each of a plurality of, illustratively, 6 MHz bandwidth television signals are modulated onto adjoining radio frequency RF channel slots and transmitted to receivers within the CATV system. These channels are typically arranged side by side in a consecutive manner with little or no gaps from the first RF carrier (Lowest Frequency) to the last RF carrier (Highest frequency).
Specifically, a first step in the CATV FDM process comprises modulating each of a plurality of baseband information signals (e.g., each television signals) onto a standard intermediate frequency to produce a corresponding plurality of IF modulated signals. In the United States, the IF is typically 43.75 MHz for analog video and 44 MHz for digitally modulated video or digitally modulated data. A second step in the CATV FDM process comprises upconverting each of the plurality of IF modulated signals onto a corresponding plurality of respective RF carrier signals, which are then transmitted. Thus, the CATV FDM process utilizes, for each baseband information signal, a respective IF modulator and a respective RF upconverter.
Therefore, it is seen to be desirable to provide a reduced cost method and apparatus for providing a plurality of information streams to receivers within an information distribution system. Moreover, it is seen to be desirable to specifically reduce the amount of processing (and associated circuitry) required to prepare and transmit the plurality of information streams, such as within a CATV system.
The disclosure describes a method and apparatus for modulating a plurality of information signals onto respective unique intermediate frequency (IF) carrier signals, summing the respective IF carrier signals to provide a stacked IF carrier signal and upconverting the stacked IF carrier signal to a radio frequency (RF). Advantageously, the invention reduces the cost and complexity of modulation and frequency conversion processes in those systems requiring a multiplicity of audio, video or data carriers placed side by side or in a consecutive order within a defined frequency spectrum.
Specifically, an upconverter according to the invention comprises: a plurality of modulators for modulating respective information signals onto respective intermediate frequency (IF) carrier signals, the respective IF carrier signals being separated by a predetermined frequency; a summation module, for summing the plurality of IF carrier signals to produce a stacked IF carrier signal; a first mixer, for spectrally shifting to a first frequency range, the stacked IF carrier signal, the first frequency range comprising frequencies greater than frequencies of the IF carrier signals; and a second mixer, for spectrally shifting to one of a plurality of spectral portions within a second frequency range, information within the first frequency range, the second frequency range comprising frequencies less than the first frequency range.